precision mediump float;
varying float vElevation;// （-1，1） 应该转为0-1的颜色区间



void main(){

    float f_opacity = (vElevation+1.0)/2.0;

    gl_FragColor = vec4(1.0*f_opacity, 1.0*f_opacity, 1.0*f_opacity, 1.0);


}
